Since being lighter and less brittle than inorganic lenses, and being dyeable, in recent years, plastic lenses have been increasingly in demands for optical elements such as glass lenses and camera lenses.
Among properties which have been required for those plastic lenses, a high refractive index and a high Abbe number are required as optical properties, and high heat resistance and low specific gravity are required as physical properties.
At present, as a resin which has satisfied those requirements mentioned above to some extent, for example, a polythiourethane resin which contains sulfur atoms therein has been known. Since having a high refractive index, superior impact resistance, and the like, the polythiourethane resin has been regarded as a well-balanced resin (for example, referred to Japanese Unexamined Patent Application Publication No. 63-46213).
However, although a higher refractive index and a higher Abbe number have been further required, it has been very difficult to improve the above two requirements at the same time since in the relationship between the two properties described above, as the refractive index is increased, the Abbe number is decreased. Hence, researches have been aggressively carried out so that the refractive index is increased while the decrease in Abbe number is suppressed.
Among the researches mentioned above, a method using a thioepoxy compound may be a typical proposal (for example, referred to WO-89/10575 and Japanese Unexamined Patent Application Publication No. 9-110979).
According to the methods mentioned above, while having a relatively high Abbe number, the resin is able to have a high refractive index. However, due to variation in quality of polymerizable compounds used as a raw material, which occurs in industrial manufacturing thereof, resins formed by the methods described above may have unstable refractive indexes or hues in some cases, and in addition, optical strain may be liable to be generated in some cases. In the case in which the refractive index varies, a problem may arise in that when a resin is used for a lens, power of lens may vary. In the case in which the hue varies, a problem may arise in that when a resin is used for glass lenses, a pair of lenses may be formed which have different colors from each other. Among resins used for optical applications, a resin having optical strain is rejected as a defective product. Accordingly, when optical strain becomes liable to be generated, the production yield of the resin is significantly decreased, resulting in industrial disadvantages. Hence, reasons causing the variations in refractive index, hue, and optical strain must have been clearly understood.